1. Field of the Invention
The present invention relates to a quantum-well type semiconductor laser device, and more specifically to a quantum-well type semiconductor laser device having a multi-layered quantum-well layer.
2. Description of Related Art
A multi-layered quantum-well structure is introduced to a semiconductor laser device in order to reduce threshold current, to increase quantum efficiency, to enable functions at high temperatures and high speed modulations. In this connection, if discontinuity between conduction bands of the quantum-well layer and of a barrier layer is small, electrons thermally distributes not only in the quantum-well layer but also in the barrier layer, so that characteristics of the semiconductor laser device is not improved, despite of the multi-layered quantum-well structure. Therefore, in a prior art, it is preferable that there is a large energy level difference between forbidden bands of the quantum-well layer and of the barrier layer.
On the other hand, it has been studied applying a distortion quantum-well structure to the semiconductor laser device in which the quantum-well is constituted by a semiconductor having larger lattice parameters than a substrate. The distortion quantum-well structure contributes change of structure of a valence band so as to reduce an effective mass of the lowest energy level hole. In order to obtain this effect of the distortion quantum-well structure, discontinuity between the valence bands should become large so as to increase difference between energy levels of the lowest level positive hole and an excitation level positive hole.
However, if there is a large energy level difference between forbidden bands of the quantum-well layer and of the barrier layer so as to have a large discontinuity of the valence bands, positive holes are strongly trapped in the quantum-well. In case of the multi-layered quantum-well structure, this causes localization of positive holes in which positive holes are concentrated at a quantum-well near a p-clad layer so that a little positive holes are implanted in quantum-wells far from the p-clad layer. The quantum-wells in which a little positive holes are implanted do not contribute gain so that the characteristics of the laser are not improved. Namely, the effect of the quantum-well and the localization of the positive holes are trade-off.
In prior arts, it has been tried to reoptimize the energy level difference between the forbidden bands of the quantum-well layer and of the barrier layer. For example, Takaoka and Kushibe reported that a characteristic temperature of quantum efficiency of a 1.3 .mu.m band InGaAsP quantum-well laser was optimized when its InGaAsP barrier layer composition had a forbidden band wave length of 1.13 .mu.m (Proc. 54th Conf. J. Appl. Phys. Lecture No. 28p-H-3, pp. 1024). Ogita et al. reported that a characteristic temperature of a 1.3 .mu.m band InGaAsP distortion quantum-well laser became maximum when its InGaAsP barrier layer composition had a forbidden band wave length of 1.1 .mu.m (Proc. 54th Conf. J. Appl. Phys. Lecture No. 28p-H-5, pp. 1025). Kito et al. reported that a relaxation-vibration frequency of a 1.3 .mu.m band InGaAsP distortion quantum-well laser was optimized when its InGaAsP barrier layer composition had a forbidden band wave length of 1.05 .mu.m (Proc. 54th Conf. J. Appl. Phys. Lecture No. 28p-H-6, pp. 1025).
In addition, in order to prevent the localization of the positive holes, it has been tried to form thin film barrier layers so as to increase possibility of migration of the positive holes to adjacent quantum-wells by tunnel effect. For example, Aoki et al. reported that a relaxation-vibration frequency of an InGaAs/InGaAsP quantum-well laser became twice when its barrier layer thickness was reduced from 10 nanometers to 5 nanometers (Proc. 51th Conf. J. Appl. Phys. Lecture No. 26a-R-6, pp. 914). Yamada et al. reported that the thinner the barrier layer was, the more differential gain of a 1.3 .mu.m band InGaAsP quantum-well laser was increased when its barrier layer thickness was changed from 3 nanometers to 10 nanometers (Proc. 54th Conf. J. Appl. Phys. Lecture No. 28p-H-10, pp. 1027).
There is proposed another way to improve characteristics of a quantum-well laser device by Kasukawa et al (Japanese Patent Application Laid-open No. 4-120786). In Kasukawa et al., the quantum-well laser device comprises a GRIN-SCH (Graded Refractive Index-Separate Confinement Hetero) structure in which optical confinement layers having graded compositions are disposed on and under a quantum-well active layer so that distribution of optical electric field and confinement of electrons are optimized.
However, in the above reoptimization, the energy level difference between forbidden bands of the quantum-well layer and of the barrier layer is decreased so as to reduce the quantum-well effect. In case of the thin film barrier layers, electrons become three-dimensional so that a density of states is reduced so as to decrease gain. In addition, a band structure of positive holes is changed so as to reduce the quantum-well effect so that the quantum-well effect and the localization of the positive holes become trade-off in this case. In the GRIN-SCH structure, although the whole threshold carrier density is lowered, transfer of positive holes is not improved so that the problem of the localization of the positive holes is not resolved.
In order to improve characteristics of the semiconductor laser device, the trade-off between the quantum-well effect and the localization of the positive holes should be cancelled.